FR2843390A1 - Production of lactide, useful for producing polylactide, includes treating crystallization mother liquors with water to precipitate pre-purified lactide - Google Patents

Abstract

Production of lactide comprises evaporating an aqueous solution of lactic acid or a lactic acid derivative, depolymerizing the resulting oligomers, condensing crude lactide from the vapor phase, crystallizing the crude lactide to obtain purified lactide, treating the crystallization mother liquors with water to precipitate pre-purified lactide, and drying the pre-purified lactide. Production of lactide comprises: (a) evaporating an aqueous solution of lactic acid or a lactic acid derivative to produce oligomers with a molecular weight of 400-2000, a total lactic acid equivalent acidity of 119-124.5% and an optical purity corresponding to 90-100% L-lactic acid; (b) feeding the oligomers and a depolymerization catalyst into a depolymerization reactor to produce a lactide-rich vapor phase and an oligomer-rich liquid residue; (c) condensing the vapor phase to obtain crude lactide in liquid form; (d) crystallizing the crude lactide to obtain purified lactide with a residual acidity below 10 meq/kg, a water content below 200 ppm and a meso-lactide content below 1%; (e) subjecting the crystallization mother liquors to extractive crystallization in aqueous media with control of the geometry of the crystals formed and with segregation of the lactide suspension towards the solid phase and of impurities towards the liquid phase; (f) separating and draining the crystals to obtain a wet cake rich in lactide crystals; and (g) drying the wet cake to obtain pre-purified lactide. Independent claims are also included for: (1) production of lactide by: (a) evaporating an aqueous solution of lactic acid or a lactic acid derivative to produce oligomers with a molecular weight of 400-2000, a total lactic acid equivalent acidity of 119-124.5% and an optical purity corresponding to 90-100% L-lactic acid; (b) feeding the oligomers and a depolymerization catalyst into a depolymerization reactor to produce a lactide-rich vapor phase and an oligomer-rich liquid residue; (c) condensing the vapor phase to obtain crude lactide in liquid form; (d) subjecting the crude lactide to extractive crystallization in aqueous media with control of the geometry of the crystals formed and with segregation of the lactide suspension towards the solid phase and of impurities towards the liquid phase; (e) separating and draining the crystals to obtain a wet cake rich in lactide crystals; (f) drying the wet cake to obtain pre-purified lactide; and (g) crystallizing the pre-purified lactide to obtain purified lactide with a residual acidity below 10 meq/kg, a water content below 200 ppm and a meso-lactide content below 1%; and (2) production of polylactide by polymerizing lactide produced as above.

Description

i Introduction Up to now, a multitude of production processes

  and / or purification of lactide and polylactide (PLA) have been described in the literature. However, it must be noted that even if their scientific interest is undeniable, the overwhelming majority of these remain laboratory procedures which cannot be exploited industrially. Indeed, they have recourse either to very specific apparatuses (if not unique) not having an industrial size equivalent which makes their extrapolation and / or piloting too risky or complicated; either at too low productivity and / or the significant use of consumables preventing any

  economically profitable exploitation of the process.

  However, although it comes from a renewable raw material (not dependent on petroleum) and benefits from a property of biodegradability making it possible to envisage it as one of the solutions to the growing problem of waste, PLA does not

  will find its salvation only within the framework of a cost price comparable to those currently in force within the petrochemical polymers of the sector of the commodities.

  However, two processes from the state of the art

  could meet these requirements.

  The first is described in US 5,274,073 of Gruber et al. Gruber et al. envisage an integrated process for the synthesis of PLA from a (more or less pure) solution of lactic acid and / or one of its esters comprising: 1. in one or two stages, evaporation of the water free and part of the water bound so as to produce an oligomer with a molecular mass of between 100 and 5000 uma; 2. a mixture of the depolymerization catalyst with the oligomer, followed by a thermo-cracking of the mixture with production of lactide in vapor form; 3. selective condensation of the vapors followed by fractional distillation making it possible to recover a purified lactide; and 4. polymerization of the purified lactide by opening

cycle to obtain PLA.

  The second is described in US 5,521,278 from O'Brien et al. O'Brien et al. envisage an integrated process for the synthesis of a purified lactide for PLA starting from an aqueous solution of lactic acid containing at least 50% by weight of lactic acid comprising: 1. an evaporation of free water and a small part of the water bound so as to produce an oligomer 20 comprising a number of monomeric units (n) of between 2 and 8; 2. further evaporation characterized by a larger diffusion surface for the polymer and making it possible to obtain an oligomer comprising a number of monomeric units (n) between 8 and, steps 1. & 2. are made in devices whose texture is characterized by a low iron content; 3. a mixture of the depolymerization catalyst, free of alkali metals, with the oligomer followed by thermocracking at a temperature below 240 ° C. of the mixture with production (a) of a vapor phase containing lactic acid, water, lactide and entrained heavy oligomers and (b) a liquid phase containing the heavy oligomers; 4. extraction of the fraction in vapor form (a) so that its residence time in the cracking zone is less than 15 seconds; 5. selective condensation of the vapors followed by fractional distillation making it possible to recover, by intermediate extraction, a pre-purified lactide in liquid form; and 6. a melt crystallization of the pre-purified lactide so as to leave a fraction of purified lactide characterized by a residual acidity of less than 6 meq / kg. Although these two processes seem interesting, they have certain imperfections which could jeopardize their chances of being used economically and profitably for the production of a PLA of

  quality for the convenience sector.

  If we consider the teachings of Gruber et al., We note that by this process, the quality of the lactide obtained is not sufficient to allow the synthesis of a polymer (PLA) whose mechanical properties correspond to those of the different selected applications. Indeed, it is well known to those skilled in the art that the lowest possible contents of residual water and acidity are required to obtain polymers of high molecular weight (mechanical properties), with high conversion (mechanical properties). , stability, yield) and in a short reaction time (stability

  chemical and thermal; productivity).

  However, due to the selected purification technology, namely distillation, it is impossible to obtain, on the one hand, an optically pure product [the vapor pressure curves of the various stereoisomers (L-lactide or L-LD, 5 D -lactide or D-LD, meso-lactide or meso-LD) being much too close, which is essential for applications requiring a certain crystallinity of the polymer] and on the other hand, a chemically pure product because of their own admissions, they recognize that they cannot totally avoid the opening of the lactide cycle within the distillation column and therefore, contamination of the

lactide in the system.

  If one considers O'Brien's teachings, it is noted, following the addition of an additional step, namely melt crystallization, that the optical and chemical quality of the lactide is reached. However, the new recommended process consists of a large succession of different technologies which on the one hand increases the complexity in controlling the process and on the other hand, jeopardizes its economic profitability both in terms of investments and production costs. Furthermore, if we look at all of the process steps (evaporations; thermo-cracking and distillation) with the exception of melt crystallization, they are all characterized by high working temperatures which is in contradiction with the rules of the art generally recommended in the context of the synthesis of a

  thermosensitive product such as lactide.

Brief description of the invention

  In the remainder of this text, unless otherwise indicated, the percentages are expressed by weight and the molecular weights in units of atomic mass (uma). In a first embodiment, the invention consists of an integrated low temperature process for the production and purification of lactide from an aqueous solution of lactic acid or lactic acid derivatives, comprising: a) a evaporation of the free water and part of the water 10 of constitution until oligomers having a molecular mass of between 400 and 2000 uma are obtained, a total acidity in lactic acid equivalent of between 119 and 124 , 5% and an optical purity expressed in L-lactic acid of between 90 and 100%; b) a feed of the mixture comprising a depolymerization catalyst and the oligomers obtained in a), in a depolymerization reactor with production of: bl) a vapor phase rich in lactide, and b2) a liquid residue rich in oligomers; c) a selective condensation of the lactide-rich vapor (b1) with recovery in liquid form of a crude lactide free of volatile compounds; d) crystallization in the molten state of the crude lactide (c), obtaining a purified lactide fraction having a residual acidity of less than 10 meq / kg, a water content of less than 200 ppm and a content of meso-lactide less than 1%; e) an aqueous treatment of the residual fractions of the crystallization in the molten state consisting of: el) an extractive and controlled crystallization of these fractions in an aqueous medium, with control of the geometry of the crystals formed and with segregation of the suspension of lactide towards the solid phase and impurities towards the liquid phase, so as to carry out an aqueous extraction of the impurities; e2) separation of the suspension of crystals (el) 5 from the liquid phase, then draining which separates a wet cake rich in lactide crystals from a liquid phase poor in lactide and loaded with impurities;

  e3) drying the wet cake (e2) which provides the prepurified lactide.

  A second embodiment of the invention consists of an integrated low temperature process for the production and purification of lactide from an aqueous solution of lactic acid or lactic acid derivatives, comprising: a) evaporation free water and part of the water of constitution until oligomers having a molecular mass of between 400 and 2000 uma are obtained, a total acidity in lactic acid equivalent of between 119 and 124, 5% and an optical purity expressed in L-lactic acid of between 90 and 100%; b) a feed of the mixture comprising a depolymerization catalyst and the oligomers obtained in a) in a depolymerization reactor with production of: bl) a vapor phase rich in lactide, and b2) a liquid residue rich in oligomers; c) a selective condensation of the lactide-rich vapor (b1) with recovery in liquid form of a crude lactide free of volatile compounds; d) an aqueous treatment of the crude lactide resulting from (c) consisting of: dl) an extractive and controlled crystallization in an aqueous medium, with control of the geometry of the crystals formed and with segregation of the lactide suspension towards the solid phase and impurities to the liquid phase, so as to carry out an aqueous extraction of the impurities; d2) separating the suspension of crystals (dl) from the liquid phase, then draining which separates a wet cake rich in lactide crystals from a liquid phase low in lactide and loaded with impurities; d3) drying the wet cake (d2) which provides a pre-purified lactide; e) crystallization in the molten state of the prepurified lactide (d3), obtaining a fraction of purified lactide having a residual acidity of less than 10 meq / kg, a water content of less than 200 ppm and a

  meso-lactide content less than 1%.

  However, the invention also proves to be advantageous in the context of a process for producing polylactide, the phase 20 for producing and purifying lactide from an aqueous solution of lactic acid or lactic acid derivatives comprising the steps a) to e3) of the first embodiment above, to which is added a

  lactide to polylactide polymerization step.

  Obviously, in the context of a polylactide production process, in the lactide production and purification phase starting from an aqueous solution of lactic acid or lactic acid derivatives comprising steps a) to e ) of the second embodiment above, it is also advantageously possible to add a

  lactide to polylactide polymerization step.

  In the present invention, the process step which consists of an extractive and controlled crystallization in an aqueous medium of lactide fractions, with control of the geometry of the crystals formed and segregation of the lactide suspension towards the solid phase and the impurities towards the liquid phase so as to carry out an aqueous extraction of the impurities, presents particular characteristics: - this crystallization is carried out with a quantity of water as small as possible (for example 0 to 25%); - during the crystallization phase, the mixture (lactide + water) will be brought to and maintained at a temperature barely lower than its crystallization temperature (eg 5 C 15 below); - the contact time of this mixture will be reduced the most

possible (e.g. 1 to 45 min).

  The advantages of this procedure are numerous: - large crystals are obtained which have a lamellar structure, without inclusion or occlusion, and which are pure, stable and easy to handle; - a complex is also formed under these conditions (a lactide molecule + a water molecule); - little or no meso-lactide is removed by hydrolysis; - The meso-lactide is therefore recycled, which is not eliminated by hydrolysis; this recycling represents a very interesting economic advantage for the process; the formation of large crystals also promotes the transfer of impurities to the aqueous phase; - the formation of large crystals makes it possible to obtain a more effective subsequent separation and subsequent drying. The formation of these large crystals is an essential indication that the process is operating under conditions of temperature, time and amounts of water in accordance with the invention: they are a confirmation of the fact that the process is well managed according to the invention. These large crystals are formed under conditions opposite to those of mass crystallizations. Control of the

  crystallization, according to the invention, is carried out by controlling the temperature profile: no sudden drop, but obtaining and then maintaining for a certain time a temperature barely below the crystallization temperature.

  By working in this way, one finds oneself in a zone of weak supersaturation which favors and makes it possible to control the growth of the crystals. In order to further improve this control, the mixture is seeded with pure lactide crystals to minimize the formation of new nuclei. It is not easy to quantify the size of these large crystals with a lamellar structure. Size can indeed be measured along the 3 axes of the crystal, but also by the average value of the 3 measurements. The size of the crystals

  is more of a statistical phenomenon, therefore random.

  One can also quantify the size by passage through one or more sieves, by introducing a percentage of crystals passing through the sieve, the rest being refused by a particular sieve. Other measurement principles are also possible. Measurement becomes even more complicated when considering aggregates of crystals from a process

  industrial, rather than isolated and perfectly formed crystals. In the case of mass crystallization, it sometimes seems impossible to define an individual crystal.

  Finally, account must also be taken of the variations inherent in any industrial process, which will vary the sizes although all the parameters of production

are unchanged.

  The quantification of the size of these crystals appears simpler if one examines it in a comparative way. It is clear that the average size of the individual crystals obtained by the process according to the invention, whatever the operating conditions but provided that the parameters indicated above are respected (small amount of water added, temperature at barely below the crystallization temperature, longest contact time

  possible reduction) is visibly greater than the average size of individual crystals obtained by mass crystallization (following a sudden drop in temperature).

  Some measurements made in a rudimentary manner during the 2 types of production make it possible to say that if the average size by mass crystallization is 0.1 mm, then the average size by crystallization according to the invention is 0.5 mm or more. This comparison obviously only makes sense if the chemical composition of the mixture subjected to the 2 types of crystallization is close, or even identical at the start. The values given above are therefore more orders of magnitude than absolute values. Other advantageous aspects of the process mean that the starting lactic acid derivatives include the lactic acid esters or a mixture of lactic acid and one or more

several lactic acid esters.

  It should be noted that for the implementation of the invention, the crude lactide is enriched with prepurified lactide fractions originating from the aqueous treatment of the fractions

  residual crystallization in the molten state.

  Recycling represents an important aspect of carrying out the invention: the pre-purified lactide resulting from the aqueous treatment can be recycled at any level of the

production of purified lactide.

  On the other hand, it should be noted that the content of D-lactide, during the course of the process, is controlled by

  polymerization by opening of the prepurified lactide cycle.

  In addition, it will be noted that during the course of the process, the prepurified lactide has a residual water content of between 50 and 1000 ppm, a total lactide content of between 70 and 99%, a content of lactic acid and oligomers of lactic acid between 0 and 5% and a

  mesolactide content between 0 and 15%.

  Finally, for the advantageous development of the invention, the polymerization of the purified and / or pre-purified lactide comprises the steps: a) of adding a catalyst or mixture of catalysts to the lactide; b) adding optional comonomers, oligomers, prepolymers, stabilizers, fillers, reinforcing agents or polymerization moderators to the mixture (a) during the initiation of the prepolymerization

  and / or during polymerization in an extruder.

  In certain embodiments of the invention, the polymerization of the purified and / or pre-purified lactide does not

  does not require prepolymerization.

  For the process for producing lactide according to the invention or polylactide according to the invention, it should be noted that during the production and purification of lactide, the recycled fractions of lactic acid or its derivatives are introduced at the level the purification step of the lactic acid production process or

of its derivatives.

  Unlike the processes of the state of the art, in this invention, all the part intended for extracting and purifying the lactide is carried out at low temperature (below 105 ° C.), which is an important advantage in

as part of an integrated process.

  In fact, by working at low temperature, in addition to an obvious economic advantage, any risk of racemization of the products and therefore the formation of lactic units is eliminated. It is clear that for the processes of the state of the art, these D-lactic units in low concentration do not constitute a problem for the quality of the final lactide from the moment when they integrate into the process a purification unit. stereo-specific such as, for example, crystallization in the molten state (or melt crystallization). However, within the framework of an integrated process, this concentration will gradually increase and a dysfunction of the various stages of the process will be observed. Indeed, a higher content of D-lactic units would statistically generate a higher proportion of meso-lactide and D-lactide which on the one hand, would considerably affect the stability of the flows during distillation (meso-lactide much 30 less stable) and, on the other hand, would disturb the proper functioning of the melt crystallization by the presence of the racemic or DL-lactide mixture (the optical quality of the final product then no longer being ensured). It is therefore essential for these high temperature processes to permanently eliminate its D-lactic units thus reducing the overall yield of the process and at the same time its economic viability. Another advantage of the low temperature process lies in the possibility, through the extractive recrystallization step in an aqueous medium, of extracting from the main stream the rare D-lactic units generated during the first two steps of the process. Indeed, at the end of this treatment, it is possible to obtain a lactide characterized by a chemical purity sufficient to be able to be used as (co) monomer for the synthesis of PLA but of an optical purity characterized by the presence 15 joint of meso- and L-lactide. This new approach makes it possible to envisage a fully integrated process and therefore

economically viable.

  Another innovative aspect of the present invention consists in recycling all or part of the hydrolysed by-products from the different stages of the process such as the evaporation distillates, the depolymerization residue, the filtrate from the extraction at water, etc., not directly at the level of the lactide synthesis process, but rather at that of the production of lactic acid and more particularly before the purification steps thereof. In fact, by doing so, a progressive increase in the concentration of impurities such as amino acids, proteins, carbohydrates, heavy metals, aldehydes, etc., is avoided, present in small quantities in the starting material and which disturb the proper functioning (technical and economic) of the various stages of the process as well as the

purity of the final product.

  Detailed description of the invention

  Preferably, the starting mixture will be an aqueous solution of lactic acid obtained by chemical means (hydrolysis of an ester), biochemical (fermentation) or by mixing recycled fractions. Its lactic acid concentration can vary from 15 to 100% knowing that the evaporation of this free water will generate additional costs. The chemical and optical purity of the raw material is essential in order to ensure a high conversion yield within the framework of an integrated process. In fact, too low a chemical purity of the lactic acid implies the concentration of impurities in the process, which on the one hand disturbs the chemistry linked to the synthesis of the lactide (racemization, low purification yields) and on the other hand, it is necessary to install purges which affect the mass balance of the process. Likewise, an initial optical purity that is too low would cause the appearance of a relatively large quantity (statistical reality) of the other 2 diastereoisomers of lactide (meso-lactide; Dlactide), which only complicates the purification phase and increases the flux. recycling and

  purge. Although a food grade currently present on the market could be suitable, the quality corresponding to the so-called "Heat Stable" grade, well known to those skilled in the art, of optical purity 2 95% isomer L and preferably 2 98% is preferred.

  This aqueous lactic acid solution is concentrated by evaporation so as to extract the water first.

  free and thereafter part of the water of constitution.

  The elimination of this water of constitution is accompanied by the creation of ester bonds, by a so-called polycondensation reaction, which induces the formation of lactic acid oligomers. The synthesized oligomers are ideally characterized by a molecular mass of between 400 and 2000, a total acidity in lactic acid equivalent of between 119 and 124.5% and a D-lactic acid content of between 0 and 10%. This quality makes it possible, on the one hand, to avoid the problems associated with the transfer of very viscous products and, on the other hand, too high residual acidity in the product obtained at the end of the depolymerization step.

(lactide synthesis).

  Evaporation will be carried out with particular care

  to avoid on the one hand too much entrainment in lactic unit in the extracted water vapors and on the other hand, to subject the lactic acid and its oligomers to a prolonged thermal stress which would favor the reactions of racemization.

  In order to avoid prolonged thermal stress on the product, several actions can be carried out, jointly or not. The first consists in promoting the rapid extraction of the volatile compound (water) from the reaction medium, so as to

  shift the reaction equilibrium towards the formation of oligomers and thus reduce the reaction time. Vacuum and / or gas flow entrainment of the volatile compound are advantageous options for carrying out this step.

  A second consists in increasing the reaction kinetics and therefore in reducing the reaction time by adding an esterification catalyst. As the catalysis is of the acid type, different acids can be envisaged. However, it is preferable to take care not to use Lewis type acids (APTS, ZnC12, isopropyl Ti, etc.). Indeed, they act at the level of the hydroxyl group carried by the chiral carbon of lactic acid and therefore can promote racemization reactions by activating a nucleophilic substitution with inversion of configuration on the methine group. On the other hand, protonated acids of type H2SO4, H3PO4, etc., can be used, because they act on the oxygen of the carbonyl group, which should in no case promote racemization reactions. Given the acidic nature of the raw material, the catalyst may be added during the process, i.e. when the residual free acidity of

  the oligomer will no longer be sufficient to effectively activate the reaction. Depending on the type of acid selected as catalyst, neutralization could be envisaged so as to avoid degradation of the lactide 20 during the depolymerization step.

  The reaction kinetics are strongly influenced by the temperature. However, it also promotes racemization reactions which must be avoided at all costs.

  In this context, the use of temperatures below 25 190 C and a reactor that can work under vacuum or under gas flow and offering a large exchange surface and extraction volume will solve the problem. In fact, the large exchange surface will make it possible to supply the energy necessary for the reaction in a minimum of time while avoiding overheating, while the large extraction volume will favor the elimination of the

  volatile compound (water) and therefore the reaction kinetics.

  In this context, different reactors can be interesting alternatives such as, for example, falling film, forced circulation, stirred film evaporators with or without internal condenser, etc. This phase of the process could be envisaged in one or more stages in order to optimize the technology vis-à-vis on the one hand the viscosity of the fluxes present, on the other hand the lactic acid content contained in the 10 distillates and finally in the possible need to add an esterification catalyst in order to revitalize the synthesis.

  The second step consists of a catalytic and thermal depolymerization of the oligomers obtained above so as to produce a vapor phase rich in lactide.

  The use of a catalyst is essential in order to reduce the thermockraking temperature and to avoid chemical and optical deterioration of the synthesized lactide. The catalyst will be solid or liquid and of Lewis acid type such as, for example, tin octoate, tin lactate, antimony octoate, zinc octoate, etc. Its content is between 0.1 and 5 g%. Lewis acid catalysts are characterized by a relatively high charge density. However, it has been shown that the latter 25 favor racemization reactions. In this context, it is preferable to reduce the contact time between the catalyst and the oligomers as much as possible, so make sure to mix the catalyst just before its introduction.

in the reactor.

  For the same reasons, the reactor will be selected so as to maintain the mixture (oligomer / catalyst) the

  as short as possible (0 to 30 min and preferably 0 to 15 min) at the reaction temperature while providing a large exchange surface and extraction volume.

  The working temperature will be sufficient to initiate the reaction, but not too high to avoid degradation

  or a racemization of the lactide: the temperature will be between 180 and 250 C. The optimum temperature will depend on the nature of the starting oligomer (120 to 125%), the nature of the catalyst and the pressure in the system. .

  Given the chemical instability of lactide at working temperatures and in order to shift the balance of the reaction towards the formation of lactide, it is important to extract it as quickly as possible from the reaction medium. In this context, it is preferable to maintain the reaction medium under gas flow and / or under vacuum. The second option will be preferred because it also allows

  reduce the reaction temperature.

  Due to the various constraints set out above, the use of a thin layer type evaporator, such as for example a thin film, seems particularly relevant. Indeed, from this type of device, a liquid residue is extracted at the bottom composed of oligomers of high molecular masses. The latter will, after hydrolysis, be recycled taking care to carry out a pre-treatment or

  a purge making it possible to remove the deactivated catalyst.

  At the head, the lactide-rich vapor phase is directly extracted and selectively condensed at the level of a condenser maintained at a well-determined temperature. Indeed, the condenser is kept at a temperature such that on the one hand, volatile compounds such as water, most of the lactic acid and degradation products resulting from the synthesis (acetaldehyde, etc.) remain in vapor phase (while the lactide and heavy compounds are condensed) and not too low on the other hand to avoid crystallization of the lactide. Depending on the nature and the purity of the product harvested (crude lactide), this temperature will be between 70

and 125 C.

  At the end of this selective condensation, there is a crude characterized by an L-LD content greater than 85%, even 10 90%, a meso-LD content less than 7%, or even 5%, or even at 3% and a residual water content of less than 1000

ppm, even at 500ppm.

  The subsequent stage of the process consists in purifying the crude, in order to obtain a lactide of chemical and stereospecific purity sufficient for the synthesis of PLA by ring opening. Sufficient purity implies a lactide content of between 99.0 and 99.9% and preferably still between 99.5 and 99.9%, a meso-LD content of between 0 and 1% and preferably between 0 20 and 0.5%, a water content between 0 and 200 ppm and preferably between 0 and 50 ppm, as well as an acidity between 0 and 10 meq / kg and preferably between 0 and 1 meq / kg. The technology of recrystallization in the molten state (one 25 or more stages) makes it possible to achieve this quality while working at low temperature. In the context of this technology, the impure lactide obtained above is melted and undergoes controlled cooling to initiate crystallization. The impurities will be concentrated in the liquid phase. After crystallization, the liquid phase is removed by gravity, leaving crystals coated with a film of impurities. In order to eliminate it, a partial overhaul is carried out. The liquid thus obtained entrains the film and is discharged by gravity. The operation is repeated until the required purity is reached. This succession of steps can be of the static and / or dynamic type. The desired purity reached, the content of the crystallizer is

melted and recovered.

  However, the profitability of this purification step is linked to the L-LD concentration, to the nature of the chemical impurities in the feed and to the L-LD 10 concentration present in the residues of the step.

  Indeed, the nature of the impurities present in the initial feed directly influences the efficiency of the purification. Thus, a more viscous impurity will be more difficult to extract and will require several stages of purification. Similarly, the presence of acidic and aqueous impurities will promote the opening of the lactide cycle, which

  will have a direct consequence on the performance of the stage.

  On the other hand, the concentration of L-LD in the starting solution makes it possible to significantly improve the mass yield (less impurities to be extracted, less

  degradation) but also profitability (fewer purification stages). Thus according to a person skilled in the art, while with a feed containing 85% L-LD, the theoretical yield of purification by melt crystallization is 25 78.5%, it increases to 86.4% for a feed containing 90% .

  However, when the theoretical purification yield is taken into account, the factor LLD concentration in the residue must also be included. Indeed, within the framework of an integrated process, it is preferable to be able to recycle the residue as lactide upstream in the process (to enrich a fraction for example) in order to avoid having to recycle it as lactic units (result of the hydrolysis of lactide) which increases the probability of thermal degradation (increased residence time in the process) and generates significant energy expenditure. However, to be able to recycle it at the level of the 5 crystallization as lactide, its concentration must be sufficiently high (ie of the order of 70%) so as to be able to mix it with an intermediate fraction. richer and reintroduce it into the main process flow and if possible, the closest to the final purification step. Under these conditions, starting from a feed containing 85% of L-LD, the theoretical yield of purification by melt crystallization is 78.5%, if the L-LD content of the residue is 55% but it drops to 58.9% if the content of the residue is 70%. In addition, 15 with the residue loaded with L-LD at 70%, it will be necessary to feed the unit with 200 kg of product at 85% to leave only 100 kg of finished product, while with that at 55% there only 150 kg. Taking these considerations into account, it is obvious that this type of technology cannot be economically and industrially used without a pre-purification step (such as for example distillation) which will make it possible to significantly increase the purity (the

  L-LD concentration) of the starting feed.

  For this invention, by integrating into our process a technology making it possible to recover the

  lactide, in the form of lactide and not of lactic unit, of a weakly charged residue (for example of the order of 40%), the minimum being linked to the presence of a eutectic.

  In this context, recrystallization in the molten state can be controlled in a different way and makes it possible to obtain, for a feed containing 88% of L-LD, a theoretical purification yield of 87% while requiring only one 132 kg feed load per 100 kg of finished product. Taking into account these new considerations, it is obvious that this type of technology can, this time, be economically and industrially used without a high temperature pre-purification step. The technology enabling the lactide to be recovered from the residue of the recrystallization stage in the molten state will preferably take care: ò either of producing a lactide of sufficient purity 10 to be able to be treated by recrystallization in the molten state, ie d an L-LD content equal to or greater than 60%, a residual water content sufficiently low (<1000 ppm and preferably less than 400 ppm) to avoid rapid deterioration of the lactide; * or to produce a lactide of sufficient chemical purity to be able to be used directly as a monomer for the synthesis of PLA by ring opening. The technology preferentially considered for the extraction of the lactide from the residue, in the context of this invention, comprises the following steps: 1. an extractive and controlled crystallization of the residue of the crystallization in the molten state (residues), in an aqueous medium , by controlling the geometry of the crystals formed and by causing a phase segregation between the lactide (solid phase) and the impurities (liquid phase), favoring an aqueous extraction of the impurities; 2. separation of the suspension of crystals obtained in 1, into a liquid phase poor in lactide and charged with impurities and in a wet cake rich in lactide crystals; 3. drying of the wet cake obtained in 2 This technology not being stereospecific, the product 5 resulting from this stage can reach a very high chemical purity and may also contain a certain content of meso-lactide, which constitutes a very interesting for the extraction of D-lactic units from the process. The product thus obtained could be used as an additive and mixed with the purified lactide in order to control the content of D-lactic units present and thus

  play on the properties of the synthesized polymer.

  The lactide, purified and pre-purified, synthesized by the process described in the context of this invention can then either be used as an additive for food applications (for example: agent coagulating animal or vegetable proteins, preservative agent or pH regulator , baking agent for dough in bakery), or be polymerized by ring opening 20 with a wide range of catalysts including organometallic derivatives of transition metals (group

  3 to 12) or metals from groups 13 to 15.

  A preferred approach of the present invention contemplates continuous polymerization of the purified lactide through the addition of the tin octoate / triphenylphosphine binomial in

  a twin-screw extruder (reactive extrusion).

  Although a single reactive extrusion step is sufficient to succeed in synthesizing, starting from the lactide, a PLA whose mechanical properties are sufficient to be able to be used in the field of packaging and of conveniences, this perspective can generate the disadvantages following: has a low "throughput" / productivity (prohibitive depreciation costs); * a low stability range for piloting

  production units (situation of precarious equilibrium).

  This productivity is linked to the nature of the raw material supplied in the "twin-screw". In fact, the lactide is fed into an extruder maintained at a temperature much higher than its melting temperature. However, beyond this melting temperature, its viscosity is practically zero. Consequently, a large part of the reactor is used outside of its optimal operating conditions for: * degassing and bringing the raw material to reaction temperature (heating by calender) during the feeding of a lactide in solid form; * promote the spread of synthesis

(homogenization of the mixture).

  It is only when the viscosity is sufficient that the extruder can effectively accelerate the reaction thanks to an optimal mixture with high viscosity as well as by an additional energy supply linked to

friction phenomena.

  On the other hand, the lack of viscosity in the first

  part of the machine makes the system much more sensitive to possible fluctuations in one of the production parameters (feed rate, catalyst concentration, viscosity (pressure) at the head of the die, etc.).

  In this context, it would be wise to include a first continuous prepolymerization step which would be carried out in any reactor capable of: melting (and degassing) the lactide; * add the active principle (catalyst and possibly co-catalyst) and any additives (comonomers, oligomers, prepolymers, stabilizers, fillers, reinforcing agents, polymerization moderators); * homogenize the active principle and the additives in the molten lactide and maintain the mixture at the polymerization temperature; * initiate polymerization so as to obtain a product of sufficient viscosity to be able to be treated effectively during the second step (molecular weight between 5,000 and 50,000); * feed the product continuously in an extruder with the possibility of adding any other additives (comonomers, oligomers, prepolymers, stabilizers, fillers, reinforcing agents,

  polymerization moderators) and homogenize them.

  The second step will be carried out in a twin screw extruder.

  On the other hand, since the synthesis in an extruder is continuous, it would be preferable, in order to ensure homogeneity of the PLA produced, to carry out the prepolymerization step also continuously. In this context, once the lactide has melted (and degassed), the technologies of static mixing exchangers (type SMXL from Sulzer or equivalent), 30 static mixing reactors (type SMR from Sulzer or equivalent), twin-screw reactors type List-ORP or List CRP might be fine. The advantage of this type of technology lies, among other things, in: * the narrow distribution of residence times 5 (homogeneity of the product fed into the twin-screw extruder, narrow polydispersity); * the high efficiency of mixing and dispersion for fluids of high viscosity or having a large difference in viscosity (homogenization of the catalyst or additives within the monomer); * the large heat exchange capacity (accelerate or

control the reaction).

  The PLA produced in the context of this invention will be either a homopolymer (for example synthesis starting from pure L15 lactide), or a copolymer (for example synthesized starting from lactide containing a proportion of meso-lactide,

or additives).

  Notes: 1. The process described considers lactic acid as a raw material. However, this succession of steps can be perfectly applied to the esters of lactic acid such as methyl lactate, ethyl lactate, isopropyl lactate, butyl lactate, etc. 2. In the context of the use of a lactic acid ester as a raw material, the oligomerization step will imperatively require the use of a transesterification acid catalyst type para-toluenesulfonic acid (APTS), octoate tin, sulfuric acid, etc. 3. The process described considers only the L isomer of lactic acid, but it is obvious that it can also be taken into account for the other isomer, with

know D-lactic acid.

  A preferential description of the process which is the subject

  of the present invention is described below in

reference of figure 1.

  The aqueous lactic acid solution is supplied by line 1 and can be continuously mixed with the hydrolyzed juice 10 supplied by line 2001 and coming from the hydrolysis tank 2000. However, a preferential option consists in recycling the hydrolyzed juice directly at the purification stages of the lactic acid production process via the line 2002 so that it can remove impurities such as amino acids, proteins, ions

  metallic, etc. A hydrolysis residue, preferably in solid form, can be extracted via the 2003 line, which makes it possible to purge the system of insoluble matter.

  The hydrolysis tank is only shown diagrammatically by a tank, but several tanks can be envisaged depending on the concentration and the destination of the recycled fractions. The mixture is fed continuously via line 2 in a preheater 100 which brings the mixture to the temperature required for the evaporation of water, that is to say between 50 and 150 C. It is possible to 'Continuously adding to the mixture, via line 121, an esterification catalyst stored in the tank 120. When adding a catalyst, the preheater 100 will preferably be designed so as to be able to heat and homogenize the mixture. In the evaporator 200 which can work under vacuum, at atmospheric pressure or under slight pressure, the majority of the free water and part of the water of constitution is continuously extracted in vapor form via the line 202 and condensed 210. In depending on the lactic acid content of the condensates, the latter are sent either to the hydrolysis tank 2000 via line 211, or as additional water in the extractive crystallization tank 700 via line 212 or, all

  simply eliminated via line 213.

  The concentrated lactic acid extracted continuously via line 10 201 and characterized by an average molecular mass of between 100 and 600 is fed continuously into a preheater 250 which brings the concentrated lactic acid to the oligomerization temperature, c that is to say between 80 and 180 C. It is possible to add to the mixture, via line 15 261, an esterification catalyst stored in the tank 260. When adding a catalyst, the pre -heater 250 will preferably be designed so as to be able to heat and homogenize the mixture. In the oligomerization reactor 300 which can work under vacuum, at atmospheric pressure or under slight pressure, a little free water and a majority of water of constitution is extracted in vapor form via line 302 and condensed 310. The condensates are sent to the hydrolysis tank 2000 via line 311. This step will preferably be carried out under vacuum without however reaching a pressure of less than 40 mbar absolute so as to accelerate the reaction kinetics and reduce the working temperature while avoiding too much production of

cyclic dimer.

  The oligomers extracted, via line 301, and characterized

  with a molecular weight between 600 and 2000 are fed continuously in a preheater / mixer 400.

  This preheater / mixer allows the homogenization of the continuously supplied depolymerization catalyst, at a concentration ranging from 0.2 to 5%, via line 521 and stored in tank 520 and, to bring the catalyst oligomer mixture 5 to at a temperature between 150 and 250 C (the exact temperature being a function of the molecular mass of the oligomers). It is possible that a neutralizing agent must be added to the oligomers in order to interrupt the activity of the esterification catalyst before incorporation of the depolymerization catalyst, but

  this step has not been shown in FIG. 1. It is also possible that the catalyst added during the oligomerization step is suitable for the "back biting" reaction and in this context, any addition of catalyst 15 is less, even superfluous.

  The catalytic depolymerization reactor 500 which is supplied via line 401 with an oligomer / catalyst mixture is controlled so as to promote the "back biting" reaction which generates the lactide. In this context, the temperature will be between 180 and 250 ° C., the pressure between 0.1 and 40 mbar absolute and the residence time of the mixture under the reaction conditions between 0 and 30 min, preferably between 0 and 15 min. On the one hand, a liquid residue (at working temperature) rich in oligomers is extracted from the depolymerization reactor 500, which is sent via line 502 to the hydrolysis tank 2000 and, on the other hand, a rich vapor phase. in lactide via the

line 501.

  The liquid residue collected at the bottom of the reactor is characterized by a molecular weight on average equal to or greater than that of the starting mixture 401 and by a catalyst concentration greater than that of the mixture

starting 401.

  The vapor phase extracted at the head of reactor 500 and rich in lactide 501 is selectively condensed at the level of a condenser 510 so as to maintain the volatile compounds such as water, lactic acid and degradation products resulting from the synthesis. , etc., in vapor form 513 and recovering the lactide and the heavier compounds in liquid form (crude lactide) 511. At the end of this selective condensation, the crude lactide is characterized by an L-LD content greater than 85% or even 90%, a low meso-LD content of less than 7% or even 5% and even even 3% and a residual water content of less than 1000 ppm or even 500 ppm. The condensing temperature is carefully adjusted in

  function of the pressure prevailing in the system and so as to avoid solidification of the lactide. It will be between 70 and 125 C. The volatile compounds extracted via line 513 are in turn condensed 550 and transferred, via line 551, to the hydrolysis tank 2000.

  The crude liquid lactide is fed via line 511 into a recrystallization unit in the molten state 600 or the purification takes place in one or more stages according to a static and / or dynamic process at low temperature, below 105 ° C. , so as to recover, via line 601, a pure lactide in liquid form. The latter is characterized by a lactide content of between 99.0 and 99.9% and preferably still between 99.5 and 99.9%, a meso-LD content of between 0 and 1% and preferably between 0 and 0.5%, a water content between 0 and 200 ppm and preferably between 0 and 50 ppm, as well as an acidity between 0 and 10 meq / kg and preferably between 0 and 1 meq / kg. During this purification step, two types of residue are generated. The first, extracted via line 603, contains a sufficient residual L-LD content so that it can be mixed with the crude lactide obtained from the selective condensation step 511. We consider a sufficient residual L-LD content as being between 60 and 99%. The second residue (drain) extracted via line 602 contains a residual LLD content of between 80% and 35% and is sent in liquid form to the extractive crystallization unit 700. In this unit, the drain is mixed with a aqueous phase supplied via line 702 with a water content which can range from 0 to 40%. As already announced, the aqueous phase supplied can come from the condensates of the evaporation step via line 212 or at least in part from the subsequent step of drying the pre-purified lactide via line 904. The temperature of the mixture is then reduced so as to avoid excessive supersaturation, so as to control the geometry of the crystals formed and to promote phase segregation between the lactide (phase

  solid) and impurities (liquid phase).

  The crystal suspension thus obtained is then transferred, via line 701, to a solid / liquid separation unit 800 in order to obtain, on the one hand, a phase

  liquid low in lactide and charged with impurities which will be sent via line 802 to the hydrolysis tank 2000.

  On the other hand, a wet cake rich in lactide crystals is recovered, characterized by a free water content of between 0 and 10%, a total lactide content of between 60 and 99%, a content of lactic acid and oligomers d lactic acid between 0 and 5%, and a

  mesolactide content between 0 and 15%.

  The wet cake is then fed, via line 801, to a low temperature dryer 900 (product temperature below 45 ° C.) in order to avoid the melting of the mesolactide, which will reduce the residual water content and bring it to a value between 1000 and 50 ppm. Depending on the purity of the pre-purified lactide extracted from the dryer via line 901 and liquefied in a heater 910 from which it will be extracted via line 911, it will either be mixed, via line 913, with the product fed to the stage 1 of the recrystallization stage in the molten state, either supplied directly to one of the intermediate stages of the recrystallization stage in the molten state (not shown), or finally mixed, via line 912 , with purified L-LD from the recrystallization step in the molten state 601 to then be polymerized. The mixture between the pre-purified lactide 912 and the purified L-LD 601 will be adapted so as to control the content of D-lactic unit 20 (originating from the meso-lactide) present in the final polymer. The purified L-LD 601 or the mixture of purified L-LD 601 and pre-purified lactide 912 is mixed with an active ingredient and brought to the polymerization temperature which can be between 120 and 220 C in a prepolymerization reactor 1000. The active principle or catalyst is stored in tank 1020 and fed via line 1021. Its concentration will be managed so as to maintain the monomer / catalyst ratio between 500 and 10,000, the exact content being a function of the type of polymer desired. The catalyst mentioned above can also correspond to the mixture of a catalyst with a cocatalyst such as for example, the binomial octoate tin / triphenylphosphine. The product from the prepolymerization reactor can already consist of a prepolymer characterized by a molecular weight of between 10,000 and 50,000. The latter is supplied via line 1001 to a polymerization reactor 1100 which will preferably be of the twin-screw extruder type in order to continue and finalize the polymerization. The polymer resulting from this step 1101 is characterized by a molecular mass which can be between 40,000 and 350,000 and a conversion greater than 95% or even 98%. Whether before the exchanger mixer or at the level of the polymerization reactor, comonomers, copolymers or additives (thermal stabilizer, catalytic deactivators, fillers or reinforcing elements) can be mixed with the lactide flow, but this

  approach is not shown in fig. 1.

  Another preferred approach of the present invention consists in sending the crude lactide 511 resulting from the selective condensation via the line 512 to the extractive crystallization unit 700. In this unit, the crude lactide is mixed with a supplied aqueous phase via line 702 with a water content ranging from 0 to 40%. The temperature of the mixture is then lowered so as to avoid excessive supersaturation, so as to control the geometry of the crystals formed and to promote phase segregation between the lactide (solid phase) and

impurities (liquid phase).

  The suspension of crystals thus obtained is then transferred, via line 701, to a solid-liquid separation unit 800 in order to obtain on the one hand a phase

  liquid low in lactide and charged with impurities which will be sent via line 802 to the hydrolysis tank 2000.

  On the other hand, a wet cake rich in lactide crystals is recovered, characterized by a free water content of between 0 and 10%, a total lactide content of between 60 and 99%, a lactic acid content and 5 oligomers of lactic acid between 0 and 5%, and a

  mesolactide content between 0 and 15%.

  The wet cake is then fed, via line 801, to a low temperature dryer (900) which will reduce the residual water content and bring it to a value between 1000 and 50 ppm.

  Depending on the purity of the pre-purified lactide extracted from the dryer via line 901, and liquefied in a heater 910 from which it will be extracted via line 911, it will either be fed in the recrystallization step in the molten state 15 via line 911, or mixed, via line 912, with the purified L-LD from the recrystallization step in the molten state

  601 to then be polymerized.

  The liquid pre-purified lactide is fed via line 911 into a recrystallization unit in the molten state 600 20 o the purification takes place in one or more stages according to a static and / or dynamic process at low temperature, below 105 C, so as to recover, via line 601, a pure lactide in liquid form and characterized by a lactide content of between 99.0 and 99.9% and more preferably between 99.5 and 99.9%, a meso-LD content of between 0 and 1% and preferably between 0 and 0.5%, a water content of between 0 and 200 ppm and preferably between 0 and 50 ppm, and an acidity of between 0 and 10 meq / kg and preferably between 0 and 1 30 meq / kg. During this purification step, two types of residue are generated. The first, extracted via line 603, contains a sufficient residual L-LD content so that it can be mixed with the crude resulting from the selective condensation step via 511. A sufficient residual L-LD content is considered to be between 60 and 99%. The second residue (drain) extracted via line 602 5 contains a residual L-LD content of between 80% and% and is sent in liquid form to the

extractive crystallization 700.

Examples

  a) This example illustrates the importance of recycling the by-products of the synthesis of lactide at the level of the lactic acid purification unit and not of the oligomerization step.

  A stock of lactic acid oligomers was fed to a depolymerization unit at one month intervals in order to confirm or deny the consistency of the results and therefore, the possibility of recycling the D-lactic units directly in the lactide synthesis process.

  During the storage period, the oligomer was kept fluid in a closed enclosure, with stirring and at a

temperature of 140 C.

  For the depolymerization, the oligomer was mixed for 2% 25 of its weight with tin octoate and, fed (25-30 kg / h) in a thin layer evaporator maintained at 235 C and a surface equal to 2 m2. The steam generated (lactide

  impurity) is condensed and the product obtained weighed in order to determine the productivity of the system but also analyzed in order to determine its selectivity.

  Table I: Characterization of the oligomer Effectiveness of depolymerization and Characterization of the oligomer Freshly prepared 1 month of aging Free acidity (g%) 9.8 10.5 Total acidity (g%) 122.4 122 L isomer content 97.6 90 ( %) Appearance Amber Very dark brown (black) Characterization of the effectiveness of depolymerization Freshly prepared 1 month aging Productivity (kg / h) 24 10 Conversion (%) 80 40 L-lactide content 87.8 77.9 (g%) Meso- content lactide 4.8 14.8 (g%) On the basis of the results given in table I, it is very clearly noted that the optical quality as well as the productivity decreases sharply. Prolonged maintenance of the lactate units at relatively high temperature results in progressive degradation of the latter. The degradation products generated strongly disturb the lactide synthesis reaction. In this context, recycling to level 10 of the oligomerization of the lactate units contained in the by-products having undergone thermal stress without prior purification risks strongly disturbing the

productivity of the process.

  b) Example demonstrating the importance of incorporating

  water extraction on the process yield.

  A stirred and heated reactor using 2 electric resistors (1.2 kW and 2.3 kW) was supplied with 20 liters of lactic acid marketed by GALACTIC under the label "heat stable" and characterized by a concentration of 90 % and an L-isomer content of 97.6%. The temperature of the heating elements and within the liquid is regulated so as to avoid any deviation greater than 20 C and the maximum temperature not exceeding 160 C. In order to facilitate the rapid extraction of the volatile compound, the unit is placed progressively under vacuum, the pressure varying between atmospheric pressure and 150 mbar. To avoid excessive entrainment of lactic acid in the distillates, the reactor is surmounted by a column with a height of 0.9m and a section of 0.09m filled with Rashig rings (10 X 10 mm). A temperature sensor placed at the head of the column makes it possible to control the temperature of the vapors and if necessary to reduce the heating power in order to avoid a

too much training.

  After 7 h of reaction, 6.3 kg of distillates characterized by a total acidity of 3.3% were collected as well as 17.4 kg of an oligomer characterized by a total acidity of 122.2%; a molecular weight of 1345 and a content of

isomer L of 97.3%.

  To the oligomer obtained above and maintained under stirring at a temperature of 120 ° C. is added 3% by weight of tin octoate. The mixture is fed at a flow rate of 3 kg / h in a thin layer evaporator of the thin-film type in stainless steel 316 with a surface of 0.2 m 2 whose walls are heated by an oil circulation whose temperature is maintained at 220 -230 C. The vapors generated are condensed in a condenser with a surface area of l m2 in stainless steel 316 whose temperature of the "refrigerant" fluid is maintained between 5 80 and 90 C. The entire unit is controlled under a pressure between 5 and 10 mbar absolute. The crude lactide is collected at the outlet of the condenser at a flow rate of 2.45 kg / h at an L-lactide content varying between 85 and 92%

  and a mesolactide content varying between 3 and 7%.

  A sample of the crude lactide obtained above (800 g) containing 86.4% L-LD, 4.8% meso-LD, and a residual acidity of 310 meq / kg is introduced into a crystallizer consisting of a vertical tube stainless steel 1 m long and 30 mm in diameter. The double jacket of the tube is supplied with heat transfer fluid by a thermostatically controlled heating group for controlling the crystallization, sweating or recasting phases. This crude is melted

at 105 C.

  Then, crystallization is initiated on the wall by a gradual decrease in the temperature of the heat transfer fluid present in the jacket. To avoid occlusions and inclusions within the pure crystals, this temperature drop will be 2 to 8 C / h. Part of the crude is crystallized on the walls, while the central part contains the liquid phase (drain) containing the

majority of impurities.

  Once the heat transfer fluid is brought to 60 C, the phase

liquid is extracted by gravity.

  The crystals are still covered with a film of impurities 30 which the sweating step must remove: the surface of the tube will be very gradually heated (from 60 to 98 ° C.) so as to melt the surface of the crystals of lower purity , because their melting point is lower than that of the pure product. Finally, the crystallizer is brought (at 10 C / min) to the melting of the product (97-102 C) in order to liquefy the whole harvested by gravity (melt). A final product that must meet the specifications of a lactide for synthesis of PLA, will undergo several stages

  successive purification by the same procedure.

  Table II shows the enrichment of the intermediate fractions as well as the mass yield (Yield) in overall LLD of the operation.

TABLE II

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2 3 4

FEED MELT DRAIN MELT MELT MELT

L-LD (%) 86.4 97 55.9 99 99.8 99.9

meso-LD

4.8 2 10.3 0.7 0.2 0.1

(%) acidity

520 120 - 56 18 7

(meq / kg) Yield in L100 - 19 - - 81

LD (%)

  The L-LD and meso-LD contents are determined by GC 15 after silylation of the carboxylated compounds. The acidities are titrated by potentiometry with tetrabutylaminehydroxide (TBAH) in an anhydrous solvent. The water contents have been

determined by Karl Fisher.

  The drains from the first stages were mixed so as to obtain a mixture containing 55.9% of L-LD and 9.8% of meso-LD. This product will undergo a pre-purification, before being mixed with the product resulting from the depolymerization for purification by recrystallization in the molten state,

  according to the procedure described above.

  To the 750 g of crude at 90 ° C. are added 25% by weight of cold water. The mixture is quickly brought to its crystallization temperature and will remain there for 30 min in order to promote nucleation and then the growth of the crystals (by sowing pure lactide crystals). Then the

  temperature is gradually lowered to 25 C.

  The mixture is then drained at 1,500 rpm and 367 g of large white crystals (about 0.4 mm) are harvested and dried under vacuum at 45 C. Analysis of this product after

drying is shown in Table III.

TABLE III

After drying

L-LD (%) 93.8

meso-LD 6.1 (%) water (ppm) 440

  The dried product resulting from this treatment after being mixed with the product resulting from the depolymerization, will undergo several stages of purification by recrystallization in the molten state, according to the procedure described in Example a.

  Table IV shows an increase in yield and

  the effectiveness of purification in the molten state.

* TABLE IV

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2 3 4

FEED MELT DRAIN MELT MELT MELT

  L-LD (%) 88.5 98.3 55 99.5 99.85 99.95

meso-LD

5.2 2.3 12.4 0.5 0.1 <0.1

(%) acidity

290 90 - - - 3

(meq / kg) Yield in L100 - 14 - - 86

LD (%)

  Note that the product resulting from the pre-purification in aqueous phase can be recycled at any level of the sequence of

  purification by crystallization in the molten state.

  c) Example demonstrating the possibility of obtaining a polymerizable product at the end of the aqueous pre-purification step. A crude lactide sample containing 79.1% L-LD,

  9.2% meso-LD, will undergo an aqueous prepurification treatment.

  To the 1,520 kg of crude at 80 ° C. are added 25% by weight of cold water. The mixture is quickly brought to its crystallization temperature and will remain there for 30 min. In order to promote the growth of the crystals, seed is carried out using pure lactide crystals. Then the

  temperature is reduced to 25 C.

  The mixture is then drained and 915 g of large white crystals (about 0.65 mm) are harvested and dried.

  The analysis of the dried product is shown in Table V. TABLE V After drying

L-LD 95.2

  (%) meso-LD 4.5 (%) Acidity 0.2 (%) water 200 (ppm) A small amount (5g) of the dried product resulting from this treatment was mixed with 5g of L-LD obtained by crystallization at the molten state (see Table IV, Stage 4: L5 LD 99.95%; acidity 3 meq / kg; water 47 ppm). This mixture has

  was introduced into a test tube with nitrogen sweeping.

  After solubilization of the mixture (100 ° C.) a solution of tin octoate was added so as to respect a monomer / catalyst molar ratio of 4500. Once the solution is well homogenized, it is immersed in a bath

  of oil whose temperature is thermostatically controlled at 180 C.

  After one hour of synthesis, the test tube is removed and broken so as to recover a very rigid and opaque polymer. The polymer obtained was analyzed by GPC in chloroform at 35 C: its weight-weighted molecular weight distribution is 68,000 (Mw with a PS calibration corrected on an absolute basis using a universal calibration (KPS = 1.67 10-4 , aPS = 0.692, KPLA = 1.05 10-3,

aPLA = 0.563).

  d) Example demonstrating the efficiency of the process starting from a crude lactide synthesized from an ester

lactic acid.

  20 liters of ethyl lactate sold

  by the company GALACTIC under the label "Galaster EL 97" and characterized by a concentration of ethyl ester of 97% is supplied to the installation described in example b.

  In order to allow the transesterification reactions, para-toluenesulfonic acid is added as catalyst at a concentration of 0.5% by weight. The temperature of the heating elements and within the liquid is regulated so as to avoid any deviation greater than 20 C and the maximum temperature not exceeding 175 C. To facilitate rapid extraction of the volatile compound and to avoid excessive entrainment d 'ester in distillates,

we will act as in example b.

  After 10 h of reaction, 7.8 kg of distillates characterized by an ethyl lactate content of 3% were collected as well as 12.6 kg of an oligomer characterized by a mass

  molecular of 960 and an L-isomer content of 97.1%.

  The oligomer obtained above is treated as in Example b, but 1.5% by weight of tin octoate is added, then the flow rate of the mixture is fixed at 2 kg / h, while the temperature of the fluid " "is kept between

  and 95 C. The crude lactide is collected at the outlet of the condenser at a flow rate of 1.78 kg / h at a content of Llactide varying between 73 and 78% and a content of mesolactide varying between 2 and 5%.

  A sample of the crude lactide obtained above (750 g) containing 75. 3% of L-LD, 2.3% of meso-LD, and a residual acidity of 83 meq / kg is treated according to a procedure

  identical to that exposed in example b.

  Table VI shows the enrichment of the intermediate fractions as well as the mass yield in overall L-LD of the operation.

TABLE VI

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2 3

FEED MELT DRAIN MELT MELT

L-LD (%) 75.3 98.4 43.7 99.4 99.8

meso-LD

2.3 0.3 5.1 <0.1 <0.1

(%) acidity

83 27 - - 4

(meq / kg) Yield in L100 - 26.7 - 73.0

LD (%)

  The L-LD and meso-LD contents are determined by GC after silylation of the carboxylated compounds. The acidities are titrated by tetrabutylamine hydroxide (TBAH) potentiometry in an anhydrous solvent. The water contents have been

determined by Karl Fisher.

  The drain from the first stages were mixed so as to obtain a mixture containing 42.3% of L-LD and 5.2% of meso-LD. It will undergo a pre-purification, before being mixed with the product resulting from the depolymerization for a purification by recrystallization in the identical molten state.

in example b.

  To the 1.050 kg of crude at 80 ° C., 25% by weight of cold water is added and the procedure described in example b was repeated.

  The mixture is then drained and 397 g of large white crystals (approximately 0.85 mm) are harvested and dried under vacuum at 45 C. The analysis of this product after drying is shown in Table VII. TABLE VII After drying

L-LD (%) 92.8

  meso-LD 4.3 (%) water (ppm) 385 The dried product resulting from this treatment after being mixed with the product resulting from depolymerization, will undergo several stages of purification by recrystallization in the state

  melted, according to example b.

  Table VIII shows an increase in yield and efficiency of melt purification.

TABLE VIII

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2 3

FEED

MELT DRAIN

MELT MELT

L-LD (%) 79.2 99.1 42.7 99.8 99.9

meso-LD

2.5 0.3 5.9 <0.1 <0.1

(%) acidity

87 20 - 9 1

(meq / kg) Yield in L100 - 20.2 -

LD (%)

  Note that the product from the aqueous pre-purification can be recycled at any level of the purification sequence by crystallization in the molten state.

Claims (14)

claims   1. A method for producing and purifying lactide, characterized in that, starting from an aqueous solution of lactic acid or lactic acid derivatives, the steps include: a) evaporation of the free water and of part of the water of constitution until oligomers having a molecular mass of between 400 and 2000 uma, a total acidity in lactic acid equivalent of between 119 and 124.5% and an optical purity are obtained expressed as L-lactic acid between 90 and 100%; b) a feed of the mixture comprising a depolymerization catalyst and the oligomers obtained in a), in a depolymerization reactor with production of: bl) a vapor phase rich in lactide, and b2) a liquid residue rich in oligomers; c) a selective condensation of the vapor rich in lactide 20 (b1) with recovery in liquid form of a crude lactide free of volatile compounds; d) crystallization in the molten state of the crude lactide (c), obtaining a purified lactide fraction having a residual acidity of less than 10 meq / kg, a water content of less than 200 ppm and a content of meso-lactide less than 1%; e) an aqueous treatment of the residual fractions of the crystallization in the molten state consisting of: el) an extractive and controlled crystallization of these fractions in an aqueous medium, with control of the geometry of the crystals formed and with segregation of the lactide suspension towards the solid phase and impurities towards the liquid phase, so as to carry out an aqueous extraction of the impurities; e2) separating the suspension of crystals (el) from the liquid phase, then draining which separates a wet cake rich in lactide crystals from a liquid phase poor in lactide and loaded with impurities; e3) drying of the wet cake (e2) which provides the pre-purified lactide.   2. Method for producing and purifying lactide, characterized in that, starting from an aqueous solution of lactic acid or lactic acid derivatives, the steps include: a) evaporation of the free water and d '' a part of the water 15 of constitution until obtaining oligomers having a molecular mass between 400 and 2000 uma, a total acidity in lactic acid equivalent between 119 and 124, 5% and an optical purity expressed in L-lactic acid between 90 and 100%; b) a feed of the mixture comprising a depolymerization catalyst and the oligomers obtained in a) in a depolymerization reactor with production of: bl) a vapor phase rich in lactide, and b2) a liquid residue rich in oligomers; c) a selective condensation of the lactide-rich vapor (b1) with recovery in liquid form of a crude lactide free of volatile compounds; d) an aqueous treatment of the crude lactide from (c) consisting of: dl) an extractive and controlled crystallization in an aqueous medium, with control of the geometry of the crystals formed and with segregation of the lactide suspension towards the solid phase and impurities to the liquid phase, so as to carry out an aqueous extraction of the impurities; d2) separation of the suspension of crystals (dl) from the liquid phase, then draining which separates a wet cake rich in lactide crystals from a liquid phase poor in lactide and loaded with impurities; d3) drying the wet cake (d2) which provides a pre-purified lactide; e) a crystallization in the molten state of the prepurified lactide (d3), obtaining a fraction of purified lactide having a residual acidity of less than 10 meq / kg, a water content of less than 200 ppm and a   meso-lactide content less than 1%.   3. A method of producing polylactide, characterized in that the steps for producing and purifying lactide from an aqueous solution of lactic acid or lactic acid derivatives include: a) evaporation of the free water and a portion of the water 20 of constitution until oligomers having a molecular mass of between 400 and 2000 uma, a total acidity in lactic acid equivalent of between 119 and 124.5% and a purity are obtained. optic expressed as L-lactic acid between 90 and 100%; b) a feed of the mixture comprising a depolymerization catalyst and the oligomers obtained in a), in a depolymerization reactor with production of: bl) a vapor phase rich in lactide, and b2) a liquid residue rich in oligomers; c) a selective condensation of the lactide-rich vapor (b1) with recovery in liquid form of a crude lactide free of volatile compounds; d) a crystallization in the molten state of the crude lactide (c), obtaining a fraction of purified lactide having a residual acidity of less than 10 meq / kg, a water content of less than 200 ppm and a meso content -lactide less than 1%; e) an aqueous treatment of the residual fractions of the crystallization in the molten state consisting of: el) an extractive and controlled crystallization of these fractions in an aqueous medium, with control of the geometry of the crystals formed and with segregation of the suspension of lactide towards the solid phase and impurities towards the liquid phase, so as to carry out an aqueous extraction of the impurities; e2) separation of the suspension of crystals (el) from the liquid phase, then draining which separates a wet cake rich in lactide crystals from a liquid phase poor in lactide and loaded with impurities; e3) drying the wet cake (e2) which provides the pre-purified lactide;   f) polymerization of lactide into polylactide.   4. A method for producing polylactide, characterized in that the steps for producing and purifying lactide from an aqueous solution of lactic acid or lactic acid derivatives include: a) evaporation of the water free and of part of the water of constitution until oligomers having a molecular mass of between 400 and 2000 uma, a total acidity in lactic acid equivalent of between 119 and 124.5% and a optical purity expressed as Llactic acid between 90 and 100%; b) a feed of the mixture comprising a depolymerization catalyst and the oligomers obtained in a) in a depolymerization reactor with production of: bl) a vapor phase rich in lactide, and b2) a liquid residue rich in oligomers; c) a selective condensation of the lactide-rich vapor (b1) with recovery in liquid form of a crude lactide free of volatile compounds; d) an aqueous treatment of the crude lactide from (c) 10 consisting of: dl) an extractive and controlled crystallization in an aqueous medium, with control of the geometry of the crystals formed and with segregation of the lactide suspension towards the solid phase and impurities to the liquid phase, so as to carry out an aqueous extraction of the impurities; d2) separating the suspension of crystals (dl) from the liquid phase, then draining which separates a wet cake rich in lactide crystals from a liquid phase poor in lactide and loaded with impurities; d3) drying the wet cake (d2) which provides a pre-purified lactide; e) crystallization in the molten state of the purified pre-lactide (d3), obtaining a fraction of purified lactide having a residual acidity of less than 10 meq / kg, a water content of less than 200 ppm and a meso content -lactide less than 1%;   f) polymerization of lactide into polylactide.   5. Method according to any one of the claims   above, characterized in that the starting lactic acid derivatives include the lactic acid esters.   6. Method according to one of claims 1 to 4,   characterized in that the starting lactic acid derivatives comprise a mixture of lactic acid and a   or more lactic acid esters.   7. Method according to any one of the claims   above, characterized in that the crude lactide is enriched with pre-purified lactide fractions from the aqueous treatment of the residual fractions   of crystallization in the molten state.   8. Method according to any one of claims   previous, characterized in that the prepurified lactide from the aqueous treatment can be recycled at any level of the production of purified lactide.   9. Method according to any one of the claims   above, characterized in that the content of lactide during the course of the process is controlled by ring-opening polymerization of the pre-purified lactide.   10. Method according to any one of the claims   above, characterized in that the prepurified lactide has a residual water content of between 50 and 1000 ppm, a total lactide content of between 70 and 99%, a content of lactic acid and lactic acid oligomers of between 0 and 5% and   a meso-lactide content of between 0 and 15%.   11. A method of producing polylactide according to claim 3 or 4, characterized in that the polymerization of the purified and / or prepurified lactide comprises the steps: a) of adding a catalyst or mixture of catalysts to the lactide; b) initiating the prepolymerization with the addition of optional comonomers, oligomers, prepolymers, stabilizers, fillers, reinforcing agents or polymerization moderators to the mixture (a); c) polymerization in an extruder with addition   optional comonomers, oligomers, prepolymers, stabilizers, fillers, reinforcing agents or polymerization moderators.   12. A method of producing polylactide according to claim 3 or 4, characterized in that the polymerization of the purified and / or prepurified lactide comprises the steps: a) adding a catalyst or mixture of catalysts to the lactide; b) polymerization in an extruder with addition   optional comonomers, oligomers, prepolymers, stabilizers, fillers, reinforcing agents or polymerization moderators in the mixture (a).   13. Process for producing polylactide according to   Claims 3, 4, 11 or 12, characterized in that during the purification and the production of polylactide, the recycled fractions of lactic acid or its derivatives are introduced at the stage of   purification of the lactic acid production process or its derivatives.   14. A lactide production process according to claim 1 or 2, characterized in that during the production and purification of lactide, the recycled fractions of lactic acid or its derivatives are   introduced in the purification step of the process for producing lactic acid or its derivatives.